I am currently using "ICMP using Windows DLL". You are correct that switching to a different engine causes the existing trace to resume.
I do get "There are no receivers" errors a couple times a day, although they do not occur (at least, not logged) at the time the tracing stops. For example, the last error occured about an hour before the trace stopped.
Regarding the CPU usage: after some testing, I do not believe it was a particular version of PP, rather network and operating system changes at my office. I checked all the way back to about version 2.30 and they all exhibit the same behavior. Around the time it seemed to start, we had gone from Windows 2000 to Windows XP SP1 and began sitting behind an ISA server. I do have the proxy information set up in PP and I do see "You have the latest pre-release version of PingPlotter" when I click "Check Now".
I checked a Win2000 machine that's not behind the ISA server (same speed as my office PC) and PP exited in about two seconds, taking up all of the CPU during that time. All of the PCs behind our ISA server are WinXP so I can't test one or the other at this time.